Multilevel converters are found in many high power applications in which medium to high voltage levels are present in the system. By virtue of their design, multilevel converters share the system voltage (line-to-line or phase voltages) eliminating the need of series connection of devices. These converters may be connected in Delta or Wye in a variety of well-known topologies. These topologies (and variants thereof) can be used for high-voltage direct current (HVDC) and flexible alternating current transmission system (FACTS) applications.
Modular multilevel converters (also called chain-link converters) are often used because of their high efficiencies, their modularity and scalability, as well as for their ability to produce voltage waveforms with low harmonic content which effectively reduce the need for large alternating current (AC) filters. Several modular multilevel converter topologies exist, e.g. M2LC (also called MMLC and MMC), in particular in flexible alternating current transmission system (FACTS) applications, high voltage direct current (HVDC) applications but also in motor drives etc.
Two of the main parameters in the selection among the various alternatives of converters are the cost and the losses. Both parameters are related to the total silicon area used in the converter which is affected by the voltage and current ratings. In addition to steady-state balanced operation of the converter, the designer must take into account the unbalance in the three-phase system. This unbalance results in negative sequence currents and voltages that need to be compensated by the converter. The net effect is that the current rating of the valves (or the total number of cells) will increase due to the zero sequence current or voltage that needs to be injected to compensate for the unbalanced condition. The consequence of this higher number of cells is a higher cost and higher losses.
The problems of higher cost and higher losses associated with negative sequence conditions can be mitigated by including degrees of parallelization into multi-level converter configurations for FACTS applications. Parallel connected sub-converters provide common storage elements which facilitate energy exchange between phases in the converter. This reduces the total stored energy in the converter and avoids overrating due to zero sequence to voltage or current.
The Institute of Electrical and Electronics Engineers (IEEE) article “A large power, low-switching frequency Voltage Source Converter for FACTS applications” by Javier Chivite-Zabalza et al. discloses a converter with parallel 3-level (3-L) neutral point clamped (NPC) inverters. The converter combines four three-phase 3-L NPC inverters that share a common direct current (DC) bus. The twelve resulting converter poles are combined in parallel pairs by means of inter-phase transformers (IPTs, also sometimes called inter-cell transformers, ICT) to obtain two sets of three-phase systems. A problem with this topology is that an intermediate transformer is needed to obtain the required voltage and the intermediate transformer adds significant cost to the converter arrangement, particularly in industrial applications where connection voltages are typically low enough to avoid transformer connection. Another disadvantage with the topology is that it requires a series connection of cascaded intermediate transformers to further increase the output voltage.